1. Field of the Invention
This invention relates generally to a tunneling magnetoresistive (TMR) sensor and methods of making the same, and more particularly relates to a method of forming a barrier layer of the TMR sensor which includes a three-step barrier-layer formation process.
2. Description of the Related Art
FIG. 1 is a cross-sectional illustration of a tunneling magnetoresistive (TMR) sensor 100. TMR sensor 100 may include a tantalum (Ta) seed layer 102, an antiferromagnetic (AFM) platinum-manganese (Pt—Mn) pinning layer 104, a ferromagnetic (FM) cobalt-iron (Co—Fe) keeper layer 106, a ruthenium (Ru) spacer layer 108, an FM cobalt-iron (Co—Fe) reference layer 110, an insulating aluminum-oxide (Al—O) barrier layer 112, FM cobalt-iron/nickel-iron (Co—Fe/Ni—Fe) sense layers 114, and copper/tantalum (Cu/Ta) cap layers 116. Sense layers 114 may be referred to as free layers, and keeper and reference layers 106 and 110 may be referred to as first and second pinned layers, respectively. Such a TMR sensor 100 differs from a commonly used giant magnetoresistive (GMR) sensor in that barrier layer 112 replaces a conducting copper (Cu) spacer layer. In contrast to the GMR sensor which exhibits GMR effects upon applying a sense current in a direction parallel to film planes, TMR sensor 100 exhibits TMR effects upon applying the sense current in a direction perpendicular to film planes.
In TMR sensor 100 of FIG. 1, antiferromagnetic/ferromagnetic coupling occurs between pinning and keeper layers 104 and 106, producing a unidirectional anisotropy field (HUA). Ferromagnetic/ferromagnetic antiparallel (AP) coupling also occurs within Co—Fe/Ru/Co—Fe pinned layers 106, 108, and 110, producing a spin-flop field (HSF) and an AP saturation field (HS). Due to these fields, the magnetization of keeper layer 106 (M3) is pinned in a transverse direction perpendicular to an air bearing surface (ABS) and that of reference layer 110 (M2) is pinned in an opposite direction. The lowest of the three fields, defined as a pinning field (HP), must be high enough to ensure rigid pinning for proper sensor operation.
Ferromagnetic/ferromagnetic coupling also occurs across barrier layer 112, producing a ferromagnetic (FM) coupling field (HF). This HF must be balanced by a demagnetizing field (HD), which is induced by the net magnetization of reference and keeper layers (M2-M3) in sense layers 114, in order to orient the magnetization of sense layers 114 (M1) in a longitudinal direction parallel to the ABS and thereby ensure optimal TMR responses. With this field balance, TMR sensor 100 exhibits a resistance of RJ+(½)ΔRT, where RJ is a junction resistance measured when M1 is parallel to M2, and ΔRT is the maximum tunneling magnetoresistance measured when M1 is antiparallel to M2. During operation of TMR sensor 100, M1 rotates in response to signal fields while M2 and M3 remain unchanged. This M1 rotation causes a change in the resistance of TMR sensor 100 by -(ΔRT/RJ) RJ sin θ1, where ΔRT/RJ is a TMR coefficient and θ1 is an M1 rotation angle.
In a prior art fabrication process of TMR sensor 100, barrier layer 112 is typically formed by depositing a metallic film and oxidizing the film in air or an oxygen gas. Optimal oxidation is essential for a TMR sensor 100 to attain good magnetic and TMR properties. Oxidation in air results in TMR sensor 100 with a junction resistance-area product (RJAJ) of beyond 1000 Ω-μm2 and a ΔRTRJ of beyond 30%. Oxidation in an oxygen gas of 10 Torr results in TMR sensor 100 having an RJAJ of beyond 10 Ω-μm2 and a ΔRTRJ of beyond 20%. Unfortunately, such TMR sensors cannot be used in practice as submicron-sized read sensors for magnetic recording at high densities, since RJAJ must be around 4 Ω-μm2 in order to prevent electrostatic discharge (ESD) damage to the sensors.
To illustrate further, FIG. 2 shows a graph 200 of ΔRT/RJ versus RJAJ for TMR sensors having various Al—O barrier layer thicknesses (δAl—O). With an optimal δAl—O of 0.90 nm, the TMR sensor exhibits an RJAJ ranging from 2.8 to 5.6 Ω-μm2 and a ΔRT/RJ ranging from 9.6 to 19.0%. A smaller δAl—O leads to a desired low RJAJ, but also an undesired low ΔRT/RJ. A larger δAl—O leads to an unacceptably high RJAJ and a low ΔRT/RJ. FIG. 3 shows a graph 300 of ΔRT/RJ versus a bias voltage (VB) for TMR sensors having various δAl—O. The thermal stability of the TMR sensor with an optimal δAl—O of 0.90 nm is characterized by a critical voltage (VC) where the ΔRT/RJ decreases to 10%. Its VC ranges from 238 to 264 millivolts (mV), indicating high thermal stability.
Accordingly, in order for a TMR sensor to perform magnetic recording at ultrahigh densities, further improvements in RJAJ, ΔRT/RJ and VC are needed.